1. Field of the Invention
This invention relates to a heterojunction type semiconductor photoelectric conversion device which is of particular utility when employed in a solar battery.
2. Description of the Prior Art
A heterojunction type semiconductor photoelectric conversion device is, in principle, provided with a semiconductor layer, a light-transparent, conductive layer deposited on the semiconductor layer to form therebetween a heterojunction and a conductive layer deposited on the semiconductor layer on the opposite side from the light-transparent, conductive layer; and the heterojunction type semiconductor photoelectric conversion device is arranged so that when light is incident to the semiconductor layer from the outside of the light-transparent, conductive layer, a photoelectric conversion function is provided by the presence of a barrier which is set up by the heterojunction.
In such a conventional heterojunction type semiconductor photoelectric conversion device, there is formed, in the barrier of the heterojunction, a layer which is formed inherently during the deposition of the light-transparent, conductive layer or the semi-conductor layer and which is formed of an oxide of the semiconductor forming the semiconductor layer and has a sufficiently small thickness to permit the passage therethrough of current. Since the semiconductor layer is usually formed of silicon, the abovesaid layer formed in the barrier of the heterojunction is usually formed of a silicon oxide.
Such an oxide layer formed in the barrier of the heterojunction has active oxygen, and hence is unstable chemically. Accordingly, the oxide layer, when heated, is liable to chemically react with the semiconductor layer, the light-transparent, conductive layer and an undesirable inpurity from the outside.
Accordingly, the conventional heterojunction type photoelectric conversion device has the defect that when it is exposed to high temperature for a long time, the thickness and height of the barrier set up by the heterojunction varies, resulting in a degraded photoelectric conversion characteristic and lowered photoelectric conversion effficiency.
In general, the oxide layer has a large energy band gap and when the oxide layer is a silicon oxide layer, the energy band gap is as large as 8 eV.
Consequently, even if the oxide layer formed in the barrier of the heterojunction is so thin that a current may pass through the oxide layer, the layer itself does not readily permit the passage therethrough of current and has high resistance.
Accordingly, the prior art heterojunction type semiconductor photoelectric conversion device has the defects of poor photoelectric conversion characteristic and very low photoelectric conversion efficiency.
Furthermore, in the conventional heterojunction type semiconductor photoelectric conversion device, even if the oxide layer is formed in the barrier of the heterojunction, the oxide layer is likely to pass an undesirable impurity from the light-transparent conductive layer or the outside thereof to the semiconductor layer and an useful impurity introduced in the semiconductor layer to the light-transparent, conductive layer.
As a consequence, the conventional heterojunction type semiconductor photoelectric conversion device is defective in that in long term use, the photoelectric conversion characteristic is deteriorated and, in particular, the photoelectric conversion efficiency is lowered.
Moreover, since the oxide layer in the barrier of the heterojunction is inherently formed, there is the possibility of the oxide layer being formed so thick as not to permit the passage of current therethrough.
Accordingly, the prior art heterojunction type semi-conductor photoelectric conversion device has the drawback that it is difficult to obtain the desired excellent photoelectric conversion characteristics and high photoelectric conversion efficiency.